A honeycomb structure is usually a ceramic product whose outer shape is a columnar shape or a prism shape. This honeycomb structure includes a plurality of cells partitioned by partition walls being made of a ceramic porous material and having a large number of pores, which has a honeycomb shape. Such a honeycomb structure, including the cells which become through channels of a gas, is often used as a filter or a catalyst carrier.
The honeycomb structure can be manufactured through extrusion forming by use of an extrusion forming machine including a honeycomb structure forming die for exclusive use (also simply referred to as die). This die is obtained by forming, in a die base body made of a pure metal, an alloy or the like, back holes (also often referred to as the introduction holes) through which a forming raw material (often referred to as the kneaded clay) is introduced, and slits which communicate with the back holes. Each of the slits has a shape matching a sectional shape of each cell, and has a width corresponding to a thickness of each partition wall of the honeycomb structure. Each of the back holes has a width larger than each slit, and is provided at a position corresponding to an intersection of the slits. According to the extrusion forming by use of such a die, the forming raw material introduced through the back holes moves toward the slits each having a small width, is extruded through slit openings, and is discharged as a honeycomb-structure formed body (often referred to as the honeycomb formed body).
Moreover, the above die is obtained by forming the above slits and back holes in the die base body which is not provided with any slit or back hole, through electric discharge machining (EDM) using an electrode, cutting, grinding, electrolytic machining or the like. For example, Patent Documents 1 and 2 disclose die manufacturing steps for obtaining a honeycomb structure including cells each having a hexagonal sectional shape. In the processes, a combtooth-like electrode (often referred to as the rib electrode) including a large number of thin flat plate-like projections adjusted to the slits (often referred to as the ribs) is used. Moreover, the slits are processed by the electricity discharged from the thin flat plate-like projections in the electrode.